OK I'll declare this issue solved. 3 days later and the alarm hasn't played up. Removing the backup battery from the siren solved the issue.
Just to summarise for other people who might encounter this problem:
-Alarm going off randomly - possibly sounding broken/quiet.
-If you can still unlock and start the engine with the alarm sounding - this is an alarm siren malfunction, likely due to a failed backup battery.
-open the siren, possibly mounted near the car battery and remove the small backup battery.

Thanks KHLow2008 for your suggestions.

Response from georgeperkins
1 question asked

You're welcome and glad to know you have fixed the problem.

Have a great day.

Answered by KHLow2008 (expert)
39,924 answers provided